GI vs GL: Why Both Matter for Blood Sugar and Portion Control

Author: Chantelle van der Merwe 

Registered Dietitian (SA) . PG Dip Diabetes Management (UK). All about Real nutrition for Real, every-day life ​

When it comes to managing blood sugar levels, the glycaemic index (GI) often gets most of the attention — but it’s only half the story. The glycaemic load (GL) brings in the missing piece: portion size. Glycaemic Index (GI) measures how quickly a carbohydrate-containing food raises blood glucose. High-GI foods like white bread or sugary cereals cause a sharp spike, while low-GI foods such as oats, lentils, and most fruits release energy more gradually.

But even the healthiest low-GI foods can raise blood sugar if eaten in large amounts — and that’s where Glycaemic Load (GL) comes in. GL considers both the quality (how fast) and the quantity (how much) of carbs you eat. This is where portion control becomes powerful. High-fiber, minimally processed carbs — like brown rice, lentils, or whole oats — are great choices. But finding your personal carb threshold is key. Everyone’s body responds differently to carbohydrate portions based on factors like insulin sensitivity, muscle mass, and activity level.

Bottom line:

• Choose high-fiber, low-GI carbs most often.
• Be mindful of portion sizes — even “healthy carbs” can impact your blood sugar if the portion is too big.
• Pay attention to how your body feels and what your glucose readings show — that’s the best guide to your personal balance between GI and portion size.

LET'S GET INTO MORE DETAIL

What Do GI and GL Actually Mean?

Think of the Glycaemic Index (GI) as a speed rating for carbohydrates — how fast they raise your blood glucose compared with pure glucose (which scores 100). Researchers developed this by feeding volunteers test portions containing 50 g of digestible carbohydrate, then tracking their blood sugar response over two hours. The resulting area under the glucose curve, expressed relative to glucose, became the GI.

In simple terms:

• Faster digestion and absorption = higher GI
• More fiber, intact structure, protein, or fat = slower rise and lower GI

However, GI alone has a blind spot: it doesn’t consider how much carbohydrate you actually eat. That’s where Glycaemic Load (GL) comes in.

The formula is simple: GL = (GI × grams of carbohydrate in your serving) ÷ 100

GL shows the real impact on blood glucose for your portion size. For example, watermelon looks “high” on GI tables (~72) but has a low GL (~5 per cup) because it’s mostly water and low in carbs. On the other hand, a big bowl of white rice combines a moderate-to-high GI with a hefty carb load — sending GL (and blood sugar) soaring. Today, international databases list thousands of foods with both GI and GL values so you can compare them side by side — and get a truer sense of how foods behave in real life.

Why GL Matters More for People with Diabetes

For anyone managing diabetes, post-meal glucose control is critical for both short-term comfort and long-term health.
While GI gives a clue, GL offers a more realistic picture, because it factors in both carbohydrate quality and portion size.
The American Diabetes Association (ADA) notes that using GI and GL can offer a modest benefit for blood sugar control beyond simply counting carbs. Meta-analyses, including Cochrane reviews, show that low-GI and low-GL diets can help lower HbA1c and reduce the risk of hypoglycemia. In other words, GL isn’t just theoretical — it’s a practical, evidence-based tool for steadier blood sugar control.

The Foods You Actually Eat: GI and GL in Everyday Portions

Let’s look at how GI and GL play out across real meals.

Breakfast: 
Oats are a good place to start. Steel-cut or traditional rolled oats have a low-to-moderate GI thanks to their structure and soluble fiber, resulting in a modest GL per serving. Adding yogurt or nuts lowers the curve even more. Instant oats, on the other hand, have a higher GI because processing breaks down the starch, allowing faster digestion.

For bread, white sandwich loaves typically test high GI, so two slices can deliver a moderate GL. Choosing dense, true whole-grain bread brings both GI and GL down.

Lunch:
A cooked cup of white rice has a moderate-to-high GI and about 40–45 g of carbs — a high GL combination that can drive post-meal spikes. Swapping rice for lentils or chickpeas dramatically lowers the impact. These legumes are classic low-GI, low-GL foods, rich in viscous fiber and resistant starch. 

If rice is a must, technique helps: cook and cool it before reheating. This increases resistant starch, which reduces glucose response — a fact confirmed in randomized crossover trials.

Fruit also shows the GI vs. GL contrast:

• A medium apple has a low GI and low GL.
• A ripe banana has a slightly higher GI and moderate GL.
• Watermelon scores high on GI but has a low GL per serving.
• Pineapple has a moderate GL, making portion size the key to keeping it blood-sugar-friendly.

Dinner
Cooking methods matter just as much as food choice. Potatoes, especially when mashed or baked hot, tend to have a high GI. The same potatoes served cooled (as in a potato salad with olive oil and vinegar) form more resistant starch, lowering their glycaemic effect.

Pasta is another example: al dente pasta has a lower GI than overcooked versions. Cooling and reheating pasta adds more resistant starch, too. Pairing pasta with plenty of non-starchy vegetables and lean protein helps reduce the overall meal GL even further.

How to Keep Both GI and GL in Check

You don’t need to memorize long tables — a few mindful habits go a long way:

• Mind your portions: Even moderate-GI foods become high-GL events if you double or triple your serving.
• Choose minimally processed carbs: Intact grains, beans, and whole fruit digest more slowly.
• Pair carbs with protein, fat, and fiber: This slows gastric emptying and blunts glucose spikes — a principle proven repeatedly in clinical studies.
• Use cooking techniques to your advantage: Cook, cool, and reheat starchy foods to boost resistant starch and flatten your post-meal curve.
• Personalize your approach: Check your glucose two hours after eating to learn how your body responds — that’s your unique threshold.

GI gives you speed. GL gives you impact.
Together, they tell the complete story. For those living with diabetes — and anyone aiming for balanced blood sugars — keeping both in mind helps make food choices that are practical, flexible, and empowering. By combining knowledge with portion awareness and high-fiber, minimally processed carbs, you can manage your glucose naturally and enjoy your meals — without restriction, but with understanding.